Definition
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Chalk is a soft, white, powdery limestone consisting mainly of fossil shells of foraminifers
From old English cealc chalk, lime, plaster; pebble, from Greek khalix small pebble, in English transferred to the opaque, white, soft limestone
Fine Grained Rock, Opaque Rock
Harzburgite is a plutonic rock of the peridotite group consisting largely of orthopyroxene and olivine
From the name of a town in Harzburg, Germany
Durable Rock, Medium Hardness Rock
Coarse Grained Rock, Opaque Rock